Cut the celery into 2-inch long pieces (thick matchsticks). Chop any celery leaves as well. Place the celery, stock, olive oil, garlic, and thyme in a medium frying pan over high heat. Season to taste with salt and pepper. Bring the liquid to a boil, then lower the heat and simmer, partially covered, until the celery is just tender, 10-15 minutes. Remove the celery and place in a serving dish. Reduce the cooking liquid over medium-high heat to a thicker glaze. Remove the thyme sprig, and pour the liquid over the celery, then sprinkle with the parmesan and serve.